Answer
Prospective:subjects were followed for a long time.
Randomized: it was random for the subjects which treatment out of the possible three they got.
Double-blind:the experimenters and the subjects did not know who got which treatment.
Placebo-controlled:they used placebo to check if the other two treatments had any effects on the subjects' conditions.
Work Step by Step
By prospective the author means that the people in the experiment were followed for a long time.
By randomized he means that it was random for everyone which treatment out of the possible three he got.
By double-blind he means that the people making the experiment and the subjects did not know who got which treatment.
By placebo-controlled he means that they used placebo to check if the other two treatments had any effects on the subjects' conditions.
